Generate the test certificates once under parallel make

ca_cert.c, server_key.c and server_cert.c each had their own rule
running tools/certs/mkcerts.sh, and the script rewrites all of the cert
files. With make -j the three rules run at the same time, overwrite
each other's keys and certificates, and can leave server_cert.c without
server_der, so build/test-wolfssl fails to link with an undefined
reference to server_der. wolfSSL's cross-library CI hits this
intermittently.

One pattern rule now names all three files as targets, which GNU make
treats as a single recipe that writes all of them, so mkcerts.sh runs
once under -j and a missing file still triggers it. .SECONDARY keeps
the generated sources from being deleted as intermediates.

Author: Frauschi

Makefile

@@ -1081,17 +1081,12 @@ build/http/%.o: build/http/%.c
 	@echo "[CC] $<"
 	@$(CC) $(CFLAGS) -c $< -o $@
 
-build/certs/ca_cert.c:
-	@echo "[MKCERTS] `dirname $@`"
-	@tools/certs/mkcerts.sh
+# A pattern rule tells make that one mkcerts.sh run writes all three files.
+build/%/ca_cert.c build/%/server_key.c build/%/server_cert.c:
+	@echo "[MKCERTS] build/$*"
+	@tools/certs/mkcerts.sh build/$*
 
-build/certs/server_key.c:
-	@echo "[MKCERTS] `dirname $@`"
-	@tools/certs/mkcerts.sh
-
-build/certs/server_cert.c:
-	@echo "[MKCERTS] `dirname $@`"
-	@tools/certs/mkcerts.sh
+.SECONDARY: build/certs/ca_cert.c build/certs/server_key.c build/certs/server_cert.c
 
 build/certs/server_key.o: build/certs/server_key.c
 	@mkdir -p `dirname $@` || true